1. Technical Field
The present invention relates to pictorial display devices such as television devices and more particularly to a method and apparatus for improving image fidelity, color fidelity, contrast ratio and brightness of front projection screens.
2. Description of Prior Art
Projection type pictorial display devices generally are classified as rear or back projection screens and front projection screens. With back projection screens, an image is displayed upon the selectively darkened front face of the screen by projection onto the back face of the screen.
In front projection devices an image is projected directly onto the front face of a screen from the front face. Back projection systems are generally preferred if space is available behind the screen since the contrast ratio with high ambient light levels is higher. Front projection screens are used in other applications in which the projectors must be placed forward and spaced from the screen such as for example in systems used for projection screens that are greater than eight feet diagonal. There is, however, in any type of large screen projection system a primary concern dealing with how to reduce manufacturing and operational costs while improving the quality of the image produced.
An ideal front projection screen would have high enough gain and wide enough angle of view that the light sources of standard color televisions/monitor screens or flat panel LCD displays could be used for the image source. Flat panel LCD screens and standard CRT television/monitor tubes have few convergent problems associated with projection systems and have long life and do not require cooling fans or expensive illumination lamps. Such prior art high gain screens that could be used with such image sources have narrow viewing angles. The ideal front projection screen would therefore appear as a solid black surface in the presence of reflected light, but would still reflect all of the light that falls on the screen from a projector. It would be desirable to keep manufacturing costs down by having as few elements or layers in the screen as possible and integrate the functions of the screen backing reflective surface refracting elements and diffusing elements.
Heretofore, prior art front projection screens have utilized high-energy image sources that have inherent convergent problems or require cooling fans and expensive lamps. Multiple CRT systems have convergent problems that can only be more apparent as the projector magnification increases.
LCD projectors have very loud fans are distracting if not contained in special sound proof rooms, thus prior art high gain retro-reflective materials have not been used because of the inherent low angle of view restrictions as noted above.
Screen gain is defined in prior art by an initial standard formed of white marble with a buff finish as a gain of 1. Prior art aluminized screens have a gain of 2-3 and a field of view of greater than 40 degrees. Glass beaded screens have a gain of 3-4 and a field view greater than 40 degrees.
High gain screens such as speed limit signs have a gain of 10 to 11 in a field of less than 10 degrees. The method of this invention provides for a screen with a gain of 30 or greater and a field of view greater than 40 degrees. The high gain and wide field of view of the method of this invention makes it possible to use image sources of a much lower intensity produce large screen front projection televisions. Prior art examples directed towards use of lenticular lenses can be seen in U.S. Pat. No. 5,625,489 which describes a front projection screen having a sheet array of lenticular lenses with the thickness of the sheet being defined as the focal point of the lenses. Light from a projection source is then focused onto to small points at the focal point of the lens on the back of the screen. A fiber optic is positioned at the focal point of each lens at the rear of the screen and then is bent in a 180-degree arc and brought back into the screen through a small hole in the sheet array between adjacent lenses. The fiber then emits the reflective light over a large angle of roughly 90 degrees from the fiber.
The screen of the present invention accomplishes concentration and remission of light in the direction of the viewer by putting a reflector at approximately half the distance to the focal point of the lenticular lens so that the light focuses on the front surface of the lens where it is re-admitted. Practice has shown that air lenticular lens interface provides a sufficient diffusion site to provide a screen with a gain of greater than 20. Curving the screen of the invention improves the gain to above 30 without occurrence of dark regions at the corners of the screen are hot spots in the center of the screens.
Close inspection of the screen of the invention reveals that many small bright regions surrounded by dark rings so that the gain of the screen is roughly the area of the dark spots divided by the area of bright spots.
The screen of the invention provides a mosaic of bright spots surrounded by dark regions which at a distance is perceived by the human eye as a continuous image of greater brightness. The method of the invention illustrates the fact that the eye perceives the intensity of the bright spots as brightness of the screen rather than the average of the brightness of the whole screen. This relationship has shown an improvement in apparent resolution of the screen which is caused by the plurality of serpentine lenticular lenses of the invention having a dimension that is at least one-third of the dimension of the pixels projected on the screen which the eye perceives as increased density of pixels as an increased resolution.
A front projection screen that provides a high gain front pictorial display is formed as a simple two element composite structure having irregular spheres or hyperbolic refracting lens components which integrally double as focusing lens components and diffusion surfaces integrally formed within the two elements of the projection screen. The focusing lens components are situated along the front face of this projection screen approximate the image to be viewed and the radius of the curvature of the focusing lens components are so dimensioned relative to the thickness of the screen that the foci of the focusing lens components are located along the front face of the screen. Refracting lens components with irregular focal distances are formed at the screen surface closest to the viewer such that irregular bright regions of light are formed on the same surface of the screen.